Monolithic image sensors comprise photodiodes and transistors formed in a silicon substrate. More specifically, such image sensors comprise an array of pixels each having a photodiode coupled to a sense node via a transfer transistor. A voltage accumulated by the photodiode during an integration period can be transferred to the sense node via the transfer transistor.
FIG. 1 illustrates a pixel circuit 100 of an image sensor, comprising a photodiode 102, coupled via a transfer transistor 104 to a sense node 106. The sense node 106 has a capacitance 107, which is, for example, formed of parasitic capacitances of the surrounding components etc. The sense node 106 is coupled to a supply voltage VRT via a reset transistor 108, which receives a reset signal RST at its gate, and has its body contact connected to ground. The sense node 106 is also coupled to the gate of a source follower transistor 110, which has its drain coupled to the supply voltage VRT and its body contact (also sometimes referred to as the bulk contact) connected to ground. The source of the source follower transistor 110 is coupled, via a read transistor 112, to an output line 114 of the pixel circuit 100. The read transistor receives at its gate a read signal RD, and also has its body contact coupled to ground.
In order to improve the image quality of an image sensor comprising an array of the pixel circuits of FIG. 1, there is a need to improve the sensitivity of the pixel circuit.